Historical Articles

*Presented
at the Fortieth Annual Convention of the American Electroplaters’ Society
June 16,1953

INTRODUCTION
Treatment of chromic acid solution by cation and anion exchange is now
an accepted process in plating plants. Costa first published1 the results
of
his preliminary
experiments on cation exchange in February, 1950. It was soon thereafter
that the authors’ company installed the first commercial scale unit
of this type at Grumman Aircraft Engineering Co., Bethpage, N. Y. In the
short span
of three years, the process has become widely recognized and appreciated.
Now, more
than a dozen plants are using this process on a commercial scale and numerous
others are evaluating it for their purposes.

The cation exchange process
was discussed2 at the 39th Annual American Electroplaters’ Society
meeting and some of its strong and weak points described. Essentially,
a cation exchanger in the hydrogen cycle is used to remove metallic cations
from chromic
acid solutions, substituting hydrogen cations and reconstituting the
chromic acid from its salts. When the cation exchanger becomes exhausted,
it is
regenerated by passing a strong acid solution through it.

Discussion
Costa reported1, upon his tests with a commercially available sulfonated
polystyrene cation exchanger. He found that in solutions containing
100 g/l of CrO3 or less, removal of metallic cations was essentially complete.
In solutions
containing above 100 g/l of CrO3, the effective capacity and degree
of completion of metal cation removal were both reduced. In solutions containing
more than
150 g/l of CrO3, he found attack upon the resin, as evidenced by formation
of
trivalent chromium. He tested anodizing baths, copper stripping baths
and chromium plating baths, both with and without dilution.

Use in Anodizing
Plants
This basic process is most applicable in anodizing plants where solutions
containing less than 100 g/l CrO3 are customarily used. Dilution
is therefore not necessary.
The acceptance of the process has been excellent and it will probably
not be long before all anodizers of appreciable size will employ
cation exchange.
At
frequent intervals, a portion of the bath is withdrawn and passed
through a cation exchange unit and then back to the anodizing bath. The cation
exchanger is operated
beyond breakthrough to exhaustion as complete removal of the cations
from the anodizing bath is not desirable. The chromic acid remaining
in the
cation exchange
bed when the bed becomes exhausted is displaced by water and returned
to the anodizing tank. The small amount of dilution which results
can
generally
be tolerated
since it makes up for evaporation and dragout losses.

Use in Electroplating
Plants
Electroplating operations, on the other hand, have necessitated a
more complex treatment method due to the inability of the cation
exchangers
to withstand
400 g/l CrO3 solutions. In 1952 the recovery of a hard
chromium plating solution was described2 at the technical
sessions of the 39th Annual Meeting of the
American
Electroplaters’ Society. In that report the authors stated
that the plating solution was diluted, passed through the cation
exchanger, and then all of
the resulting solution containing chromate was concentrated back
to plating strength
in a glass-lined evaporator.

The need for the evaporator, unless
it is also necessary to concentrate the save rinse solution,
places a very heavy capital load upon
the process. The
result
has been that many decorative chromium plating’ plants
have felt that it was better to tolerate the high metal cation
content
in their baths than
to try
to take it out. This is done in spite of the greatly increased
electrical efficiency of the treated bath. Hard-chromium platers,
however, find the process economically
feasible because they can tolerate less impurities in the bath
without an adverse effect on throwing power.

EXPERIMENTAL EVIDENCE
The desirability of treating a plating-strength bath directly
by cation exchange was obvious, so the authors undertook to
investigate the possibility
of developing
a practical process. First tests were run with a sulfonated
polystyrene cation exchanger (Permutit Q). It was found that reduction
of the metal content
in 400 g/l CrO3 solutions was possible but that
the capacity was low; above 200 g/l, there was some resin degradation
as evidenced by the reduction
of
CrO3 to Cr+++, and bleaching
and swelling of the exchanger.

Modified Resin Used
The authors, having established that this resin was not sufficiently
stable, developed several cation exchangers for handling
40 percent CrO3 solutions
which gave promise of greater oxidation resistance. One of
these identified as a modified
sulfonated polystyrene (Permutit QC) performed well when
tested. Its capacity for metals in chromic acid was almost identical
with that
of
the resin
first used,
but no physical or chemical breakdown was encountered using
400 g/l CrO3 solutions.

The solution used in these
tests contained approximately 10 g/l of copper. In typical
runs, during which the breakthrough
was
considerably overrun,
the copper
concentration was reduced by 200 per cent. No Cr+++ was
encountered in the effluent. Up to 14 cycles were run on individual resin
samples and
there
did not appear
to be any reduction in capacity.

Table
I. Comparison of Operating Methods*

Method 1

Method 2

Influent CrO3

Influent Cu

Influent Cr+++

Effluent CrO3

Effluent Cu

Effluent Cr+++

394

10.9

0

348

5.9

0

384

9.8

0

387

8.3

0

*Values
given in grams per liter.

Method # 1
Tests were run in two different manners. In the first,
several volumes of strong CrO3 solution were allowed
to percolate
through a bed of
the modified
exchanger.
The water initially in the bed was displaced to waste,
then a quantity of solution equivalent to that introduced
was
collected by draining
the bed.
This resulted
in the loss of some CrO3 held up in the modified resin.

Method #2
The second method was essentially the same except that
after introduction of the strong CrO3 solution, water
was introduced
and allowed to
percolate down
through the bed displacing the CrO3. This
resulted in the overall dilution of the influent but
recovery of
all the
chromium.
Two parallel tests
are shown in
Table I.

The mechanism of ion exchange depends
upon diffusion of a portion of each ionic constituent into the ion exchange
particle.
Initially
a
regenerated particle
is filled, much like a tiny sponge, with water. When
the
chromic acid is
introduced, part of the CrO3 enters the
particle and displaces some of the water. Thus the
exhausted resin particle contains some CrO3.
Introduction of water reverses the equilibrium and
the CrO3 is removed
from
the particles.

For Method 1, the apparent holdup
amounted to 140 grams CrO3 per liter of resin. It required an
additional
1.2 liters
of water per
liter of
the modified
resin
to recover this CrO3. To recover 811 of
the chromium in a typical experiment would result
in 25 per cent
dilution of
a 400 g/l
solution. Otherwise,
losses would approximate 10 per cent of the CrO3.
The losses
are in direct proportion
to the degree of contamination of the CrO3 solution,
since the volume treated is in inverse proportion
to the contamination

Regenerant Tests
Tests were made with the original and the modified
sulfonated polystyrene cation exchangers to determine
the effect
of regenerant upon capacity.
Sufficient confirming data were developed upon
the special resin to indicate that results
were almost identical
with those obtained for the conventional material.
Increasing regenerant dosage increased capacity,
although not proportionately. Increasing regenerant
concentration
from 10 per cent to 30 per cent decreased capacity.
Hydrochloric acid was found to be a more efficient
regenerant than sulfuric acid in terms of pounds,
although
in terms of dollars such may not be the case. One
major advantage of hydrochloric acid is that a
higher capacity is achieved. A higher resin capacity
results
in less dilution or chromate loss, depending upon
which operating method is resorted
to. Hydrochloric acid is warranted where a low
initial cost is preferred to a minimum operating
cost.

The cost of the hydrochloric acid
regenerant is
approximately 1/3 of the value of the recovered
CrO3. Thus the
treatment is economically
very attractive,
particularly
in locations where there are waste disposal problems.

Operating
Costs Data
The operating cost of treatment of a strong chromic
acid solution at any concentration is the sum
of the costs
of regenerant
chemicals, and steam
for evaporation back
to the initial concentration, plus amortization
of ion exchange unit and evaporator. The authors
surveyed
the
effect of the
concentration to which
an initial 400
g/l solution as given in Table II is diluted
upon the operating cost
of the overall treatment.
The minimum dilution possible is 25 percent
so the figure covers only lower concentrations.
Thus in
operation the
water used
to displace the CrO3 from
the modified resin
at the completion of one cycle would be used
to dilute the plating solution prior to the
next
cycle.
More
water could
obviously
be added to give
greater dilution.
Any concentration below 300 g/l can be treated.

Fig.
1, based on the data shown in Table II, indicates
that treatment should be carried
out at as high
concentration as possible. Some
items were neglected
(such as labor, electrical power and cooling
water) but these
are either minor or not changed appreciably
with concentration. If
one assumes
the bath must be
dumped when it reaches the concentrations
of cations listed above (because, for instance,
of its poor
throwing power)
the yearly
replacement cost
of chromic
acid without ion exchange would be $19,850.
Thus the treatment cost at 300 g/l of $7,700
would
be rapidly
repaid.

Anion Exchanger Stability
There has been considerable discussion
on the stability of anion exchangers
when treating
dilute chromic
acid rinse solutions. The use of ion
exchange for recovery
of chromate from rinse waters and purification
of these rinse
waters has much to recommend it. Compared
with
conventional waste
disposal
processes, it is the
only method which pays its own way
in terms of the chromic acid recovered.
Its application
automatically
results
in the formation
of demineralized
water which
is excellent for rinsing purposes.

The
authors have discussed previously3 the accelerated laboratory and plant
scale tests
that were made
to determine the stability
of a highly
basic
anion exchanger (Permatit S),
when contacting chromic acid. Table
III shows the relative stability
of that
basic anion
exchanger in acid and
alkaline solution.
At 1000 ppm CrO3 content
and below, the resin maintains its
operating capacity
exceptionally well over long periods
of time either in acid or alkaline
solutions.

Fig. 2. Stability of a highly basic anion exchanger. Life test on contact
with chromic acid.

Leakage
Long term tube tests-also have been
made at various CrO3 levels
to confirm these
findings. Fig. 2
shows the operating
capacity
of the
basic anion
exchanger when
treating a solution of 500 ppm
CrO3, 5 ppm SO4 and
10 ppm Cr+++ expressed
as per cent change in original
capacity.
It was
found that leakage
of CrO3 through
the
resin increased slightly as the
test proceeded. However, it was
always
below 10 ppm. Since
the water was recirculated
in
a closed
cycle,
the leakage did
not result in a loss in CrO3 nor
did it constitute a
waste problem. This concentration
should be satisfactory in
a final rinse
bath. A rinse water
of 500 ppm is
higher
than would occur in most operations
thus the leakage found in the test
is greater
than actually would
be encountered. The leakage
is a function
of
the influent
concentration so that in lower
concentrations the leakage would
be negligible.

Plant Scale Experience
Plant scale experience in many
locations generally has confirmed
these laboratory
results. It
has also been
found that in
several locations
where the treatment
was carried out at elevated temperatures
there was little or no harm to
the resin.

Fig. 3. Chromate recovery flow diagram.

The authors conclude
from the laboratory and numerous plant
scale installation
tests, that
the method
of operation shown in Fig.
3 is most satisfactory.
The rinse-water is withdrawn
and passed directly to the
highly basic
anion
exchanger where the chromate
is removed. No neutralization
prior to anion exchange treatment
seems necessary.
The effluent
from
the anion
exchanger
goes directly to the sulfonated
polystyrene cation exchanger
where
traces of cations are
removed and then the demineralized
water is returned to the rinse
tank.
When the anion exchanger becomes
exhausted, caustic soda solution
is sent in series
through it and
the cation
exchanger. The
effluent is
H2CrO4 which
can
be used as produced, or concentrated
for addition to the bath.
The recovery process is of
value to the plater and metal
finisher
in several
important respects.
It
eliminates a serious waste;
it recovers
a valuable
chemical; it improves the quality
of work..

MR. GILBERT (Rock Island Arsenal,
Rock Island, Ill.): I note from Table I of your paper that no mention was
made of trivalent chromium ion. That
is, by
far, the foremost contaminant of most hard-chromium plating solutions. I
wonder if
you could give us some idea of the capacity of the resins in terms of equivalents.

MR.
PAULSON: The capacity of the resins for trivalent chromium is comparable
to their capacity for copper and other cations. In treatment at high concentrations,
incomplete removal of cations is always achieved, so there is no differentiation.
In the operations where complete removal is desired, it is more difficult
to
get complete removal of trivalent chromium.

MR. GILBERT: Then, in other words,
there is bed leakage of trivalent chromium through the system.

MR. PAULSON: Yes, in all cases,
in these concentrated solutions, there is leakage of every cation.

MR. GILBERT: Could you give us some
idea of the total capacity of this new resin in terms of equivalents?

MR. PAULSON: As I said, it is comparable
to the previously used resins in dilute
concentrations and falls off in more concentrated solutions.1 2 3

MR. GILBERT:
In other words, around 2 normal?

MR. PAULSON: We have found that
normality bears a much smaller effect than the pH of the influent solution.
The pH of the influent solution is a major
criterion.
Above ,pH approximately 0.22-0.25 the capacity is rather high; whenever you
get below that pH, whether in concentrated solution or dilute solution which
contain
a small amount of contaminants, the capacity falls off.

MR. GILBERT: Under those
circumstances, would it not be true that the efficiency of the recycling
of the solution would drop with the increasing purity of solution,
In other words, the law of diminishing return would prevail.

MR. PAULSON: The
capacity falls as the concentration rises. However, as I pointed out, the
overall treatment becomes more attractive as the concentration rises.

MR.
GILBERT: I note that you recommend the use of hydrochloric acid as a regenerant.
Does this not seem to be a hazardous procedure in view of the fact that while
leakage or inadvertent introduction of sulfuric acid to the bath can be removed,
the chloride ion is much more difficult to remove from the solution?

MR. PAULSON:
We think that is a function of equipment design. With proper design, no chloride
will leak into the bath. Also, as shown in the text, normally we
do use sulfuric acid; it is only in the treatment of small baths where high
CrO3 concentrations are desirable that we recommend the use of hydrochloric
acid for
its higher resulting capacity.

MR. GILBERT: What are the concentrations
of the regenerants, sulfuric and hydrochloric acids?

MR. PAULSON: The optimum for both
is between 10 and 25 per cent.

MR. H. A. FUDEMAN
(Trico Products Corp., Buffalo, N. Y.): Have you studied the effect upon
your resins of the fluoride type catalyst baths that are sometimes
used in chromium plating?

MR. PAULSON: We have studied proprietary
baths containing large amounts of
fluoride such as are used in aluminum surface finishing and found there was
no particular effect. A considerable amount of work is being carried out
on that
problem, and as far as we .know, no deleterious effect traceable to the fluoride
ion has been found.

MR. FUDEMAN: What becomes of these
catalysts in the process?

MR. PAULSON: Sulfate,
chromate, and fluoride anions are all recovered by the anion exchangers;
they are all unaffected by the cation exchangers. There is
evidence to indicate that some: sulfate is held by the cation exchanger in
a complex with trivalent chromium, but it is a rather minor effect as far
as operating
conditions go. The same holds true for high speed self-regulating baths which
contain cations that have catalytic properties; those cations will be removed
along with copper and other cations.

MR. PAULSON: Theoretically,
QC should be more resistant. Since both are resistant to temperatures of
250° F,
the question is an academic one when treatment of chromic acid solutions is
concerned.

MR. J. M. ANDRUS (Croname, Inc.,
Chicago, Ill): Does the ion exchange system remove strontium which is used
widely in
chromium plating for self-regulating
purposes?

MR. PAULSON: It would remove some
of the strontium.

MR. GEORGE E. BEST (Mutual
Chemical Company of America, Baltimore, Md.): As far as the two resins are
concerned, Q and QC, what is the cost on a purely
relative
basis?

MR. PAULSON: The Permutit QC is
about 20 per cent higher.

MR. BEST: With reference
again to trivalent chromium, have you explored the chemical pre oxidation
of trivalent chromium prior to cation exchange treatment?

MR. PAULSON:
We have not explored that problem. However, since you must take the solution
out of the plating tank for treatment in other equipment, the conditions
are good for any oxidation process. Where the trivalent chromium is a relatively
unimportant constituent, it is probably more economical to forget about oxidation.
Where it is the main constituent, oxidation effects should be considered
along with some simple method for avoiding them.

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